Organic-inorganic hybrid perovskite solar cells (PSCs) have experienced unprecedented efficiency growth from 4% to 22% certified efficiency within several years of active research, allowing significant progress in realizing a low-cost solution-based process for photovoltaics. One of the key components of PSCs is the hole-transport layer (HTL) that is in contact with the perovskite absorber to extract the photogenerated positive charge carriers (holes). HTLs must have a relatively large conductivity, ideally with more affinity for holes than for electrons. While various HTLs ranging from small molecules, polymers, to carbon based materials, have demonstrated good performance in PSCs, 2,2′,7,7′-tetrakis(N,N-di-p-methoxyphenylamine)-9,9′-spirobifluorene (spiro-OMeTAD) has prevailed as the dominant hole-transport material for high-efficiency devices. However, pristine spiro-OMeTAD has a relatively low conductivity and moderate hole-mobility, limiting its functionality as a HTL in high-efficiency PSCs.
The conductivity of spiro-OMeTAD may be increased through p-doping, whereby additives are added to increase the charge-carrier density in the material. In organic-semiconductors, this may be achieved using chemical oxidants such as lithium salts, ionic liquids, cobalt(III) salts, F4TCNQ and SnCl4. Lithium bis(trifluoromethanesulfonyl)imide (Li-TFSI) has been used as a dopant for spiro-OMeTAD. While Li+ itself does not have a strong enough redox potential to oxidize spiro-OMeTAD, oxidation by Li-TFSI is a slow process that requires oxygen infiltration (normally from the ambient air) into the hole-transport layer. In addition, the LixOy by-products are hygroscopic compounds, potentially facilitating moisture infiltration into the perovskite layer while the spiro-OMeTAD is aging/oxidizing to improve its conductivity. The moisture ingression is a main cause of the degradation of the perovskite material and device performance. Thus, strategies for rapidly enhancing the conductivity of HTLs represent an important area in developing PSC technology.